CBSE Class 9 Answered

In humans, the respiratory system includes the nasal passages, pharynx, larynx, trachea, the primary bronchi and lungs. Within the lungs, the bronchi split further into bronchioles, alveolar ducts and alveoli. Moving down the respiratory tract starting at the trachea, the tubes get smaller and divide more and more. There are estimated to be about 20 to 23 divisions, ending up at an alveolus.

Air enters the nostrils, passes through the nasopharynx, the oral pharynx, through the glottis into the trachea, into the right and left bronchi, which branches and rebranches into bronchioles, each of which terminates in a cluster of alveoli.

Most of the respiratory tract is present mainly to conduct air to the lungs; alveoli are the actual sites of the lung that exchanges oxygen and carbon dioxide with the blood.

The respiratory tract is covered in an epithelium, the type of which varies down the tract. Most of the epithelium (from the nose to the bronchi) is covered in columnar ciliated epithelial cells, commonly called respiratory epithelium. The cilia beat in one direction, moving mucus towards the throat where it is swallowed. Moving down the bronchioles, the cells get more cuboidal in shape but are still ciliated.

Cartilage is present until the small bronchi. In the trachea they are C-shaped rings, whereas in the bronchi they are interspersed plates.

The major function of the respiratory system is gas exchange between the external environment and an organism's circulatory system. In humans, this exchange facilitates oxygenation of the blood with a concomitant removal of carbon dioxide and other gaseous metabolic wastes from the circulation. As gas exchange occurs, the acid-base balance of the body is maintained as part of homeostasis.